1. Field of the Invention
The present invention relates generally to semiconductor laser devices and more particularly relates to a semiconductor laser device which is suitable for a recording or readout light source of a recording and/or reproducing apparatus for an optical video disc, a digital audio disc and so on.
2. Description of the Prior Art
Roughly classified, conventional semiconductor laser devices are of a refractive index-guiding type and a gain-guiding type regarding its optical and carrier confinement mechanisms.
Examples of the refractive index-guiding type semiconductor laser device are shown, for example, in FIGS. 1 and 2. As shown in FIGS. 1 and 2, a GaAs substrate 1 of, for example, N type is provided. On one major surface thereof is previously formed a stripe concave portion 2 (in FIG. 1) or convex portion 3 (in FIG. 2) which extends in the one direction. On the substrate 1 which includes the concave portion 2 or the convex portion 3 are sequentially epitaxially grown an N type first cladding layer 4 made of Al.sub.x Ga.sub.l-x As, a P or N type active layer 5 made of, for example, GaAs, a P type second cladding layer 6 made of Al.sub.x Ga.sub.l-x As and a P type capping layer 7 which is useful for the ohmic contact with an electrode. Due to the forbidden band gap of each of the first and second cladding layers 4 and 6 is selected large so that a heterojunction is formed between the active layer 5 and the first and second cladding layers 4 and 6, respectively, moreover the existence of the concave portion 2 or convex portion 3 formed on the substrate 1, a stripe light emission region 8 is formed in the active layer 5 in one direction between the bent portions of the first and second cladding layers 4 and 6. Namely, the optical confinement is effected by the refractive index difference due to the above bent portions.
On the other hand, the gain-guiding type semiconductor laser device is formed as shown in FIG. 3. That is, on a substrate 1 which has a flat surface, are epitaxially grown in turn the first cladding layer 4, the active layer 5, the second cladding layer 6 and the capping layer 7 which are the same as described before. High resistance layers 9 are formed by selectively injecting, for example, protons from the above of the capping layer 7. Then, a stripe current path 10 is formed by the high resistance layers 9 and thereby the current concentration is carried out. This current concentration allows the stripe light emission region to be confined in the active layer 5.
Although the above refractive index-guiding and gain-guiding type semiconductor laser devices have respectively advantages, they have defects, respectively. More particularly, since the longitudinal mode of the refractive index-guiding type semiconductor laser device is a single mode, when this type semiconductor laser device is used as the writing or readout light source of the writing in and/or reading out apparatus for the optical video disc and so on, the laser device is easily influenced by the noise caused by the return light. On the other hand, since a so-called beam waist position exists close to the light end face of the light emission region, this refractive index-guiding type semiconductor laser device has an advantage that in the practical use the focal position can be determined with ease. Furthermore, this type laser device has such advantage that a long distance image, namely, a so-called far field pattern regarding the cross-section parallel to the junction plane is symmetrical with respect to left and right sides and that a beam spot having less distortion can be obtained with ease as the readout or writing light source in the practical use. Meanwhile, in the above gain-guiding type semiconductor laser device, there are some defects that the beam waist position exists at the position inside of the end face of the light emission region by approximately 20 .mu.m, and furthermore the far field pattern is apt to become asymetrical with respect to left and right sides. Due to the above defects astigmatism is large and the distortion of the beam spot becomes relatively large. However, in this gain-guiding type semiconductor laser device, its longitudinal mode is multimode so that this laser device is less influenced by the noises caused by the returned light.